1. Field of the Invention
The present invention relates to the field of ball valves, more particularly to disposable plastic ball valves.
2. Description of the Prior Art
Ball valves of the known design are disclosed, for example, in U.S. Pat. No. 3,550,902 and Japanese Unexamined Patent Publication (Kokai) No. 57-37160. Such ball valves include tubular caps having connecting portions for receiving ends of pipelines and abutting against outer end walls of the valve housing. On the external surfaces of the tubular caps, there are provided outwardly extended flanges facing the outer end walls of the valve housing.
The ball valve further includes cap nuts having inwardly extended flanges and internal threaded portions. Thus, firm connection between the valve housing and the tubular caps can be obtained when the internal threaded portions of the cap nuts are threaded onto the external threaded portions of the valve housing.
The ball valve disclosed in U.S. Pat. No. 3,550,902, however, has the following drawbacks:
(a) The union is merely inserted in the valve body, so when the ball is a closed position, the union and the ball tend to disconnect from the valve housing due to upstream-side fluid pressure on the ball when the downstream-side tubular cap is removed from the valve housing, e.g., so as to replace the seating ring or O-ring mounted in the union.
(b) The polytetrafluoroethylene seating rings have no resiliency, so the contact surfaces of the seating rings become worn by rotational friction of the ball. Tight sealing between the ball and the seating rings is therefore soon reduced. Thus, the seating rings must be replaced after a comparatively short period of valve use.
(c) The seating rings contact the annular grooves formed in the valve body and the union at their two sides, so there is a likelihood of the seating rings being extruded into the flow passage when the fluid pressure is applied to the seating rings through the ball.
(d) After the ball valve is connected to the pipelines, the pipelines are apt to deviate from the axis of the flow passage when excess heat or external force is applied. This deviation of the pipelines causes damage to the cap nuts and reduces the tight sealing performance between the valve housing and the tubular caps, between the valve body and the union, and between the union and the ball.
In the construction of the ball valve disclosed in Japanese Unexamined Patent Publication No. 57-37160, the union is threaded into the valve body. Therefore, if the tubular caps and the cap nuts are removed from the valve housing while the ball is in a closed position, the ball and the union do not disconnect from the valve housing due to the applied fluid pressure. As in the case of U.S. Pat. No. 3,550,902, however, the ball is disconnected from the valve housing when the union is removed from the valve body so as to replace the seating rings. In the case of this prior art, periodic replacement of worn seating rings is required. In addition this type of the ball valve has the same drawbacks (c) and (d) as the former ball valve.